High-strength steel sheet and method of manufacturing high-strength steel sheet

ABSTRACT

A steel sheet contains, by mass %, 0.03% to 0.35% C, 0.01% to 0.50% Si, 3.6% to 8.0% Mn, 0.01% to 1.0% Al, 0.10% or less P, and 0.010% or less S, the remainder being Fe and inevitable impurities. The steel sheet is continuously annealed, heated at a heating rate of 7° C./s or more in a temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600), the maximum end-point temperature of the steel sheet in an annealing furnace is 600° C. to 700° C., the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 30 seconds to 10 minutes, and the concentration of hydrogen in an atmosphere is 20% by volume or more in a heating step.

TECHNICAL FIELD

This disclosure relates to a high-strength steel sheet having excellent chemical conversion treatability and corrosion resistance after electrodeposition coating even when the content of Si or Mn is high. The disclosure also relates to a method of manufacturing the high-strength steel sheet.

BACKGROUND

In recent years, from the viewpoint of improving the fuel efficiency of automobiles and the viewpoint of enhancing the crash safety of automobiles, there have been growing demands that automobile bodies are lightened and strengthened such that automobile body materials are strengthened and are thereby gauged down. Therefore, application of high-strength steel sheets to automobiles is promoted.

Usually, steel sheets for automobiles are used after being coated. The steel sheets are subjected to a chemical conversion treatment, called a phosphate treatment, as a pretreatment for coating. The chemical conversion treatment of a steel sheet is one of the important treatments to ensure corrosion resistance after coating.

Addition of Si and Mn is effective in increasing the strength and ductility of a steel sheet. However, even when the steel sheet is annealed in a reducing N₂+H₂ gas atmosphere in which Fe is not oxidized (Fe oxides are reduced), Si and Mn are oxidized during continuous annealing to form surface oxides (such as SiO₂ and MnO, hereinafter referred to as selective surface oxides) selectively containing Si or Mn in the outermost surface layer of the steel sheet. Since the selective surface oxides inhibit formation reaction of a chemical conversion coating during a chemical conversion treatment, a fine region (hereinafter referred to as a lack of hiding in some cases) where no chemical conversion coating is produced is formed, leading to a reduction in chemical conversion treatability.

Japanese Unexamined Patent Application Publication No. 5-320952 discloses a method of forming a 20-1,500 mg/m² iron coating layer on a steel sheet by an electroplating process as a conventional technique to improve the chemical conversion treatability of a steel sheet containing Si and Mn. However, that method has a problem that an electroplating line is necessary and therefore the increase in the number of steps causes an increase in cost.

Phosphate treatability is enhanced by regulating the Mn/Si ratio as described in Japanese Unexamined Patent Application Publication No. 2004-323969 or by adding Ni as described in Japanese Unexamined Patent Application Publication No. 6-10096. However, the effect depends on the content of Si or Mn in a steel sheet. Hence, further improvements are probably necessary for steel sheets with a high Si or Mn content.

Japanese Unexamined Patent Application Publication No. 2003-113441 discloses a method in which an internal oxidation layer made of an Si-containing oxide is formed within a depth of 1 μm from the surface of an underlayer of a steel sheet by controlling the dew point at −25° C. to 0° C. during annealing such that the proportion of the Si-containing oxide in a length of 10 μm in a surface of the steel sheet is 80% or less. However, in the method disclosed in JP '441, controlling the dew point is difficult and stable operation is also difficult because an area where the dew point is controlled is based on the whole inside of a furnace. When annealing is performed by the unstable control of the dew point, the distribution of internal oxides formed in a steel sheet is uneven and unevenness in chemical conversion treatability (a lack of hiding in the whole or a portion) may possibly be caused in a longitudinal or transverse direction of the steel sheet. Alternatively, in enhanced chemical conversion treatability, there is a problem in that corrosion resistance after electrodeposition coating is poor because the Si-containing oxide is present directly under a chemical conversion coating.

Japanese Unexamined Patent Application Publication No. 55-145122 discloses a method in which an oxide film is formed on a surface of a steel sheet in an oxidizing atmosphere by increasing the temperature of the steel sheet to 350° C. to 650° C. and the steel sheet is heated to a recrystallization temperature in a reducing atmosphere and is then cooled. However, in that method, the thickness of the oxide film formed on the steel sheet surface is uneven depending on the oxidation process, oxidation does not occur sufficiently, or the oxide film is too thick so that the oxide film remains or peels off during subsequent annealing in the reducing atmosphere and therefore surface properties are poor in some cases. In an example, a technique of performing oxidation in air is described. However, oxidation in air has a problem that, for example, thick oxides are produced and subsequent reduction is difficult or a reducing atmosphere with a high hydrogen concentration is necessary.

Japanese Unexamined Patent Application Publication No. 2006-45615 discloses a method in which an oxide film is formed on a surface of a cold-rolled steel sheet containing 0.1% or more Si and/or 1.0% or more Mn on a mass basis at a steel sheet temperature of 400° C. or higher in an atmosphere oxidizing iron, followed by reducing the oxide film on the steel sheet surface in an atmosphere reducing iron. In particular, Fe in the steel sheet surface is oxidized at 400° C. or higher using a direct fired burner with an air ratio of 0.93 to 1.10 and the steel sheet is then annealed in an N₂+H₂ gas atmosphere reducing Fe oxides, whereby selective surface oxidation which deteriorates chemical conversion treatability is suppressed and an Fe oxidation layer is formed on the outermost surface. JP '615 does not particularly describe the heating temperature of the direct fired burner. When a large amount (about 0.6% or more) of Si is contained, the amount of oxidized Si, which is more likely to be oxidized than Fe, is large. Hence, oxidation of Fe is suppressed or is slight. As a result, formation of a surface Fe reduction layer after reduction is insufficient or a lack of hiding is caused in a chemical conversion coating in some cases because SiO₂ is present on the steel sheet surface after reduction.

It could therefore be helpful to provide a high-strength steel sheet having excellent chemical conversion treatability and corrosion resistance after electrodeposition coating even when the content of Si or Mn is high. It could also be helpful to provide a method of manufacturing the high-strength steel sheet.

SUMMARY

We thus provide:

-   -   (1) A method of manufacturing a high-strength steel sheet         includes continuously annealing a steel sheet containing, by         mass %, 0.03% to 0.35% C, 0.01% to 0.50% Si, 3.6% to 8.0% Mn,         0.01% to 1.0% Al, 0.10% or less P, and 0.010% or less S, the         remainder being Fe and inevitable impurities. In a heating step,         the steel sheet is heated at a heating rate of 7° C./s or more         in a temperature range corresponding to an annealing furnace         inside temperature of 450° C. to A° C. (where 500≦A≦600), the         maximum end-point temperature of the steel sheet in an annealing         furnace is 600° C. to 700° C., the transit time of the steel         sheet in a temperature range corresponding to a steel sheet         temperature of 600° C. to 700° C. is 30 seconds to 10 minutes,         and the concentration of hydrogen in an atmosphere is 20% by         volume or more.     -   (2) In the method of manufacturing the high-strength steel sheet         specified in Item (1) above, the steel sheet further contains,         by mass %, one or more selected from among 0.001% to 0.005% B,         0.005% to 0.05% Nb, 0.005% to 0.05% Ti, 0.001% to 1.0% Cr, 0.05%         to 1.0% Mo, 0.05% to 1.0% Cu, 0.05% to 1.0% Ni, 0.001% to 0.20%         Sn, 0.001% to 0.20% Sb, 0.001% to 0.10% Ta, 0.001% to 0.10% W,         and 0.001% to 0.10% V as a component composition.     -   (3) The method of manufacturing the high-strength steel sheet         specified in Item (1) or (2) above further includes performing         electrolytic pickling in an aqueous solution containing sulfuric         acid after the continuous annealing is performed.     -   (4) A high-strength steel sheet is manufactured by the method         specified in any one of Items (1) to (3) above. The amount of an         oxide of one or more selected from among Fe, Si, Mn, Al, P, B,         Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V per side is less         than 0.030 g/m² in total, the oxide being formed in a surface         portion of the steel sheet that is within 100 μm from a surface         of the steel sheet.

A high-strength steel sheet has a tensile strength TS of 590 MPa or more. The term “high-strength steel sheet” as used herein includes both a cold-rolled steel sheet and a hot-rolled steel sheet.

A high-strength steel sheet having excellent chemical conversion treatability and corrosion resistance after electrodeposition coating is obtained even when the content of Si or Mn is high.

DETAILED DESCRIPTION

Hitherto, an inner portion of a steel sheet containing oxidizable elements such as Si and Mn has willingly been oxidized for the purpose of improving the chemical conversion treatability thereof. However, this causes chemical conversion treatment unevenness or a lack of hiding on a surface because of the oxidation of the inner portion or deteriorates corrosion resistance after electrodeposition coating. Therefore, we investigated a method of solving this problem by a novel technique without being bound to conventional ideas. As a result, we found that formation of internal oxides in a surface portion of a steel sheet is suppressed and excellent chemical conversion treatability and high corrosion resistance after electrodeposition coating are achieved such that the heating rate, atmosphere, and temperature in a heating step during continuous annealing are appropriately controlled. During continuous annealing, the steel sheet is annealed such that the steel sheet is heated at a heating rate of 7° C./s or more in a temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600), the maximum end-point temperature of the steel sheet in an annealing furnace is controlled to be 600° C. to 700° C., the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is controlled to be 30 seconds to 10 minutes, and the concentration of hydrogen in an atmosphere is controlled to be 20% by volume or more in the heating step. Subsequently, a chemical conversion treatment is performed. Since the heating rate in the temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600) is 7° C./s or more, the maximum end-point temperature of the steel sheet in the annealing furnace is 600° C. to 700° C., and the concentration of hydrogen in the atmosphere in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 20% by volume or more in the heating step, the potential of oxygen at the interface between the steel sheet and the atmosphere is reduced, internal oxidation hardly occurs, and the selective surface diffusion and oxidation (hereinafter referred to as surface oxidation) of Si, Mn, and the like are suppressed.

No internal oxide is formed and surface oxidation is minimized by controlling the heating rate of such a limited region only and the concentration of hydrogen in an atmosphere, whereby a high-strength steel sheet free from a lack of hiding and unevenness and having excellent chemical conversion treatability and corrosion resistance after electrodeposition coating is obtained. The term “excellent chemical conversion treatability” refers to having an appearance free from a lack of hiding and unevenness after a chemical conversion treatment.

A high-strength steel sheet obtained by the above method includes a surface portion within 100 μm from a surface of the steel sheet. In the surface portion, formation of the following oxide is suppressed: an oxide of one or more selected from among Fe, Si, Mn, Al, P, and further B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V. The amount of the formed oxide per side is limited to less than 0.030 g/m² in total. This leads to excellence in chemical conversion treatability and a significant increase in corrosion resistance after electrodeposition coating.

Our steel sheets and methods are described below in detail. In the descriptions below, the unit of the content of each element in the steel composition is “mass percent” and is simply denoted by “%” unless otherwise specified.

First, annealing conditions which are the most important requirements and which de-termine the surface structure of a steel sheet are described. To allow a high-strength steel sheet made from steel containing large amounts of Si and Mn to have satisfied corrosion resistance, the internal oxidation of a steel sheet surface layer that may possibly be the origin of corrosion needs to be minimized. Chemical conversion treatability can be enhanced by promoting internal oxidation of Si and Mn. However, this causes deterioration of corrosion resistance as described above. Therefore, it is necessary that good chemical conversion treatability is maintained by a method other than promoting internal oxidation of Si and Mn and corrosion resistance is enhanced by suppressing internal oxidation. The potential of oxygen is reduced in an annealing step for the purpose of ensuring chemical conversion treatability and the activity of Si, Mn and the like, which are oxidizable elements, in a base metal surface portion is reduced. This suppresses the external oxidation of these elements, resulting in improved chemical conversion treatability. Furthermore, formation of internal oxides in a steel sheet surface portion is suppressed and therefore corrosion resistance after electrodeposition coating is improved.

Such an effect is obtained such that when annealing is performed in a continuous annealing line, the heating rate in a temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600) is controlled to be 7° C./s or more, the maximum end-point temperature of a steel sheet in an annealing furnace is controlled to be 600° C. to 700° C., the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is controlled to be 30 seconds to 10 minutes, and the concentration of hydrogen in an atmosphere is controlled to be 20% by volume or more in a heating step. Since heating is performed such that the heating rate in the temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600) is controlled to be 7° C./s or more, formation of surface oxides is minimized. Since the concentration of hydrogen in the atmosphere is controlled to be 20% by volume or more in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C., the potential of oxygen at the interface between the steel sheet and the atmosphere is reduced and the selective surface diffusion and surface oxidation of Si, Mn, and the like is suppressed without forming internal oxides. As a result, excellent chemical conversion treatability free from a lack of hiding and unevenness and high corrosion resistance after electrodeposition coating are achieved.

The reason why the temperature range in which the heating rate is controlled is 450° C. or higher is as described below. In a temperature range lower than 450° C., surface oxidation and internal oxidation do not occur to such an extent that the occurrence of a lack of hiding and unevenness, the deterioration of corrosion resistance, and the like are problematic. Thus, the temperature range, in which the desired effect is exhibited, is 450° C. or higher.

The reason why the temperature range is A° C. (where 500≦A≦600) or lower is as described below. In a temperature range lower than 500° C., the time for which the heating rate is controlled to be 7° C./s or more is short and therefore the desired effect is small. The effect of suppressing surface oxidation is insufficient. Therefore, A is 500 or more. The case of higher than 600° C. is disadvantageous from the viewpoints of the deterioration of annealing furnace internals (rolls and the like) and an increase in cost, although there is no problem for the desired effect. Thus, A is 600 or less.

The reason why the heating rate is 7° C./s or more is as described below. The effect of suppressing surface oxidation is recognized when the heating rate is 7° C./s or more. The upper limit of the heating rate is not particularly limited. When the heating rate is 500° C./s or more, the above effect is saturated, which is disadvantageous in terms of cost. Therefore, the heating rate is preferably 500° C./s or less. The heating rate can be adjusted to 7° C./s or more such that, for example, an induction heater is placed in an annealing furnace in which the temperature of the steel sheet is 450° C. to A° C.

The reason why the maximum end-point temperature of the steel sheet in the annealing furnace is 600° C. to 700° C. is as described below. When the maximum end-point temperature of the steel sheet is lower than 600° C., good material quality is not obtained. Therefore, a temperature range in which the desired effect is exhibited is 600° C. or higher. However, in a temperature range higher than 700° C., surface oxidation is significant and deterioration of chemical conversion treatability is serious. Furthermore, in the temperature range higher than 700° C., the effect of balancing between strength and ductility is saturated from the viewpoint of material quality. From the above, the maximum end-point temperature of the steel sheet is 600° C. to 700° C.

The reason why the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 30 seconds to 10 minutes is as described below. When the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is less than 30 seconds, target material quality (TS, El) is not obtained. When the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is more than 10 minutes, the effect of balancing between strength and ductility is saturated.

The reason why the concentration of hydrogen in the atmosphere in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 20% by volume or more is as described below. The effect of suppressing surface oxidation begins to be recognized when the concentration of hydrogen is 20% by volume. The upper limit of the concentration of hydrogen is not particularly limited. When the concentration of hydrogen is more than 80% by volume, the above effect is saturated, which is disadvantageous in terms of cost. Therefore, the concentration of hydrogen is preferably 80% by volume or less.

The compositions of the steel sheet are described below.

C: 0.03% to 0.35%

C forms martensite and the like as a steel microstructure to enhance workability. Therefore, the content of C needs to be 0.03% or more. However, when the content of C is more than 0.35%, strength increases extremely and elongation decreases, resulting in the deterioration of workability. Thus, the content of C is 0.03% to 0.35%.

Si: 0.01% to 0.50%

Although Si is an element effective in strengthening steel to obtain good material quality, Si is an oxidizable element and therefore is disadvantageous for chemical conversion treatability. Si is an element that should be avoided being added as much as possible. However, about 0.01% of Si is inevitably contained in steel. Reducing the content of Si to 0.01% or less leads to an increase in cost. From the above, the lower limit is 0.01%. On the other hand, when the content of Si is more than 0.50%, the effect of increasing the strength and elongation of steel is saturated and chemical conversion treatability is deteriorated. Thus, the content of Si is 0.01% to 0.50%.

Mn: 3.6% to 8.0%

Mn is an element effective in increasing the strength of steel. 3.6% or more Mn needs to be contained to ensure mechanical properties and strength. However, when more than 8.0% Mn is contained, it is difficult to ensure chemical conversion treatability and the balance between strength and ductility. Furthermore, there is a cost disadvantage. Thus, the content of Mn is 3.6% to 8.0%.

Al: 0.01% to 1.0%

Al is added for the purpose of deoxidizing molten steel. The purpose of deoxidizing molten steel is not achieved when the content of Al is less than 0.01%. The effect of deoxidizing molten steel is achieved when the content of Al is 0.01% or more. However, when the content of Al is more than 1.0%, an increase in cost is caused. Furthermore, the surface oxidation of Al is increased and it is difficult to improve chemical conversion treatability. Thus, the content of Al is 0.01% to 1.0%.

P≦0.10%

P is one of the inevitably contained elements. To adjust the content of P to less than 0.005%, an increase in cost may possibly be caused. Therefore, the content of P is preferably 0.005% or more. However, when more than 0.10% of P is contained, weldability is deteriorated. Furthermore, deterioration of chemical conversion treatability is significant and it is difficult to enhance chemical conversion treatability. Thus, the content of P is 0.10% or less and the lower limit is preferably 0.005%.

S≦0.010%

S is one of the inevitably contained elements. The lower limit is not particularly limited. Weldability and corrosion resistance are deteriorated when a large amount of S is contained. Therefore, the content of S is 0.010% or less.

To improve surface quality and further improve the balance between strength and ductility, the following element may be added where appropriate: an element that is one or more selected from among 0.001% to 0.005% B, 0.005% to 0.05% Nb, 0.005% to 0.05% Ti, 0.001% to 1.0% Cr, 0.05% to 1.0% Mo, 0.05% to 1.0% Cu, 0.05% to 1.0% Ni, 0.001% to 0.20% Sn, 0.001% to 0.20% Sb, 0.001% to 0.10% Ta, 0.001% to 0.10% W, and 0.001% to 0.10% V.

When these elements are added, the reason for limiting the appropriate amount of each added element is as described below.

B: 0.001% to 0.005%

The effect of accelerating hardening is unlikely to be obtained when the content of B is less than 0.001%. However, when the content of B is more than 0.005%, chemical conversion treatability is deteriorated. Thus, when B is contained, the content of B is 0.001% to 0.005%. When the addition of B is judged to be unnecessary to improve mechanical properties, B need not be added.

Nb: 0.005% to 0.05%

The effect of adjusting strength is unlikely to be obtained when the content of Nb is less than 0.005%. However, when the content of Nb is more than 0.05%, an increase in cost is caused. Thus, when Nb is contained, the content of Nb is 0.005% to 0.05%.

Ti: 0.005% to 0.05%

The effect of adjusting strength is unlikely to be obtained when the content of Ti is less than 0.005%. However, when the content of Ti is more than 0.05%, deterioration of chemical conversion treatability is caused. Thus, when Ti is contained, the content of Ti is 0.005% to 0.05%.

Cr: 0.001% to 1.0%

A hardening effect is unlikely to be obtained when the content of Cr is less than 0.001%. However, when the content of Cr is more than 1.0%, Cr is surface-oxidized and therefore weldability is deteriorated. Thus, when Cr is contained, the content of Cr is 0.001% to 1.0%.

Mo: 0.05% to 1.0%

The effect of adjusting strength is unlikely to be obtained when the content of Mo is less than 0.05%. However, when the content of Mo is more than 1.0%, an increase in cost is caused. Thus, when Mo is contained, the content of Mo is 0.05% to 1.0%.

Cu: 0.05% to 1.0%

When the content of Cu is less than 0.05%, the effect of accelerating formation of a retained γ-phase is unlikely to be obtained. However, when the content of Cu is more than 1.0%, an increase in cost is caused. Thus, when Cu is contained, the content of Cu is 0.05% to 1.0%.

Ni: 0.05% to 1.0%

The effect of accelerating the formation of the retained γ-phase is unlikely to be obtained when the content of Ni is less than 0.05%. However, when the content of Ni is more than 1.0%, an increase in cost is caused. Thus, when Ni is contained, the content of Ni is 0.05% to 1.0%.

Sn: 0.001% to 0.20%, Sb: 0.001% to 0.20%

Sn and Sb may be contained from the viewpoint of suppressing nitriding or oxidation of a surface of the steel sheet or decarburization of a region of tens of micrometers in the steel sheet surface, the decarburization being due to oxidation. Suppressing nitriding or oxidation thereof prevents the amount of martensite produced in the steel sheet surface from being reduced, thereby improving fatigue properties and surface quality. From the above viewpoint, when Sn and/or Sb is contained, the content of each of Sn and Sb is 0.001% or more. Deterioration of toughness is caused when the content of either of Sn and Sb is more than 0.20%. Therefore, the content of each of Sn and Sb is preferably 0.20% or less.

Ta: 0.001% to 0.10%

Ta forms a carbide and a carbonitride with C and N to contribute to increasing strength and also contributes to increasing yield ratio (YR). Ta has the property of refining the microstructure of a hot-rolled steel sheet. This property reduces the diameter of ferrite grains after cold rolling and annealing. Thus, the amount of C segregated along grain boundaries increases with the increase in area of the grain boundaries and a high bake hardening value (BH value) can be obtained. From this viewpoint, 0.001% or more Ta may be contained. However, containing more than 0.10% Ta causes an increase in material cost and may possibly interfere with formation of martensite in the course of cooling after annealing. Furthermore, TaC precipitated in the hot-rolled steel sheet increases deformation resistance during cold rolling to make stable actual manufacture difficult in some cases. Thus, when Ta is contained, the content of Ta is 0.10% or less.

W: 0.001% to 0.10%, V: 0.001% to 0.10%

W and V are elements forming a carbonitride and having the property of increasing the strength of steel by a precipitation effect and may be added as required. When W and/or V is added, this property is exhibited when the content of each of W and V is 0.001% or more. However, when more than 0.10% W and/or V is contained, strength is extremely increased and ductility is deteriorated. From the above, when W and/or V is contained, the content of each of W and V is 0.001% to 0.10%.

The remainder other than the above elements are Fe and inevitable impurities. If an element other than the above elements is contained, our steel sheets are not adversely affected. The upper limit of the element other than the above elements is 0.10%.

A method of manufacturing the high-strength steel sheet and the reason for limiting the method are described below.

Steel containing the above chemical components is hot-rolled, followed by cold rolling, whereby a steel sheet is obtained. The steel sheet is annealed in a continuous annealing line. Furthermore, the steel sheet is preferably electrolytically pickled in an aqueous solution containing sulfuric acid. The steel sheet is then subjected to a chemical conversion treatment. When the steel sheet is annealed, the heating rate in a temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600) is 7° C./s or more, the maximum end-point temperature of the steel sheet in an annealing furnace is 600° C. to 700° C., the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 30 seconds to 10 minutes, and the concentration of hydrogen in an atmosphere is 20% by volume or more in a heating step. This is the most important requirement. In the above, hot rolling is directly followed by annealing without performing cold rolling in some cases.

Hot Rolling

Hot rolling can be performed under usual conditions.

Pickling

Pickling is preferably performed after hot rolling. Mill scales formed on a surface are removed in a pickling step, followed by cold rolling. Pickling conditions are not particularly limited.

Cold Rolling

Cold rolling is preferably performed with a rolling reduction of 40% to 80%. When the rolling reduction is less than 40%, the recrystallization temperature is reduced and therefore mechanical properties are likely to be deteriorated. However, when the rolling reduction is more than 80%, not only rolling costs increase because of the high-strength steel sheet but also chemical conversion treatability is deteriorated because surface oxidation increases during annealing in some cases.

The cold-rolled or hot-rolled steel sheet is annealed and then subjected to the chemical conversion treatment.

In the annealing furnace, the heating step is performed such that the steel sheet is heated to a predetermined temperature in an upstream heating zone and a soaking step is performed such that the steel sheet is held at a predetermined temperature for a predetermined time in a downstream soaking zone.

In the heating step, the heating rate in the temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600) is 7° C./s or more, the maximum end-point temperature of the steel sheet in an annealing furnace is 600° C. to 700° C., the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 30 seconds to 10 minutes, and the concentration of hydrogen in the atmosphere is controlled to be 20% by volume or more as described above.

Gaseous components in the annealing furnace are nitrogen, hydrogen, and inevitable impurities. If no desired effect is impaired, another gaseous component may be contained.

The concentration of hydrogen in a temperature range other than the temperature range corresponding to a steel sheet temperature of 600° C. to 700° C., that is, a temperature range lower than 600° C. or higher than 700° C., is not particularly limited. When the concentration of hydrogen therein is less than 1% by volume, no activation effect due to reduction is obtained and chemical conversion treatability is deteriorated in some cases. The upper limit is not particularly limited. When the upper limit is more than 50% by volume, an increase in cost is caused and an effect is saturated. Thus, the concentration of hydrogen therein is preferably 1% to 50% by volume and more preferably 5% to 30% by volume. The remainder are N₂ and inevitable impurity gases. If no desired effect is impaired, a gaseous component such as H₂O, CO₂, or CO may be contained.

After cooling from the temperature range of 600° C. to 700° C., quenching or tempering may be performed as required. Conditions are not particularly limited. Tempering is preferably performed at a temperature of 150° C. to 400° C. This is because elongation tends to be deteriorated when the temperature is lower than 150° C. and hardness tends to be reduced when the temperature is higher than 400° C.

Even if electrolytic pickling is not performed, good chemical conversion treatability can be ensured. However, after continuous annealing is performed, electrolytic pickling is preferably performed in an aqueous solution containing sulfuric acid for the purpose of removing slight amounts of surface oxides inevitably formed during annealing to ensure better chemical conversion treatability.

A pickling solution used for electrolytic pickling is not particularly limited. Nitric acid and hydrofluoric acid are highly corrosive to equipment, requires caution in handling, and therefore are not preferable. Hydrochloric acid may possibly generate chlorine gas from a cathode and therefore is not preferable. Thus, in consideration of corrosiveness and the environment, sulfuric acid is preferably used. The concentration of sulfuric acid is preferably 5% to 20% by mass. When the concentration of sulfuric acid is less than 5% by mass, conductivity is low. Hence, the voltage of an electrolytic bath rises during electrolysis to increase the load of a power supply in some cases. However, when the concentration of sulfuric acid is more than 20% by mass, the loss due to drag-out is large, which is problematic in terms of cost.

Conditions for electrolytic pickling are not particularly limited. To efficiently remove surface oxides such as oxides of Si and Mn, formed after annealing and inevitably surface-oxidized, alternating electrolysis is preferably performed at a current density of 1 A/dm² or more. The reason for performing alternating electrolysis is as described below. When the steel sheet is being held as a cathode, the pickling effect is small. In contrast, when the steel sheet is being held as an anode, Fe dissolved during electrolysis is accumulated in the pickling solution and therefore the concentration of Fe in the pickling solution is increased. Hence, a problem with dry stains or the like occurs if the pickling solution is attached to a surface of the steel sheet.

The temperature of an electrolytic solution is preferably 40° C. to 70° C. The temperature of a bath is increased by heat generated by continuous electrolysis and therefore it is difficult to maintain the electrolytic solution at lower than 40° C. in some cases. From the viewpoint of the durability of a lining of an electrolytic cell, it is not preferable that the temperature of the electrolytic solution exceeds 70° C. When the temperature of the electrolytic solution is lower than 40° C., the pickling effect is small. Therefore, the temperature of the electrolytic solution is preferably 40° C. or higher.

The high-strength steel sheet is obtained as described above. The surface structure of the steel sheet is featured as described below.

In a surface portion of the steel sheet that is within 100 μm from a surface of the steel sheet, the amount of the following oxide per side is limited to less than 0.030 g/m² in total: an oxide of one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V.

In the steel sheet, which is made from steel containing large amounts of Si and Mn, it is required that the internal oxidation of a surface layer of a base steel sheet is minimized, chemical conversion treatment unevenness and a lack of hiding are suppressed, and corrosion and cracking during heavy machining are also suppressed. Therefore, the potential of oxygen is reduced in the annealing step for the purpose of ensuring good chemical conversion treatability, whereby the activity of Si, Mn, and the like, which are oxidizable elements, in a base metal surface portion is reduced. Furthermore, external oxidation of these elements is suppressed and formation of internal oxides in the base metal surface portion is also suppressed. As a result, not only good chemical conversion treatability is ensured but also workability and corrosion resistance after electrodeposition coating are enhanced. Such an effect is obtained such that in a surface portion of the steel sheet that is within 100 μm from a surface of the base steel sheet, the amount of the following oxide is limited to less than 0.030 g/m² in total: an oxide of at least one selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V. When the sum (hereinafter referred to as the internal oxidation amount) of the amounts of formed oxides is 0.030 g/m² or more, not only corrosion resistance and workability are deteriorated, but also chemical conversion treatment unevenness and a lack of hiding are caused. Even if the internal oxidation amount is limited to less than 0.0001 g/m², the effect of improving corrosion resistance and the effect of enhancing workability are saturated. Therefore, the lower limit of the internal oxidation amount is preferably 0.0001 g/m².

Examples

Our steel sheets and methods are described below in detail with reference to examples.

Hot-rolled steel sheets with a steel composition shown in Table 1 were pickled, whereby mill scales were removed. Thereafter, the hot-rolled steel sheets were cold-rolled under conditions shown in Tables 2 and 3, whereby cold-rolled steel sheets with a thickness of 1.0 mm were obtained. After the mill scales were removed, some of the hot-rolled steel sheets (a thickness of 2.0 mm) were used without being cold-rolled.

TABLE 1 Steel symbol C Si Mn Al P S Cr Mo B Nb Cu Ni Ti Sn Sb Ta W V A 0.11 0.02 4.4 0.02 0.01 0.003 — — — — — — — — — — — — B 0.02 0.02 4.5 0.03 0.01 0.003 — — — — — — — — — — — — C 0.35 0.02 4.8 0.03 0.01 0.003 — — — — — — — — — — — — D 0.13 0.11 4.6 0.02 0.01 0.003 — — — — — — — — — — — — E 0.12 0.31 4.7 0.03 0.01 0.003 — — — — — — — — — — — — F 0.11 0.49 4.5 0.03 0.01 0.003 — — — — — — — — — — — — G 0.12 0.02 3.7 0.03 0.01 0.003 — — — — — — — — — — — — H 0.11 0.03 6.2 0.02 0.01 0.003 — — — — — — — — — — — — I 0.11 0.02 8.0 0.03 0.01 0.003 — — — — — — — — — — — — J 0.12 0.02 4.6 0.29 0.01 0.003 — — — — — — — — — — — — K 0.11 0.03 4.5 0.99 0.01 0.003 — — — — — — — — — — — — L 0.12 0.02 4.6 0.02 0.04 0.003 — — — — — — — — — — — — M 0.11 0.03 4.4 0.03 0.09 0.003 — — — — — — — — — — — — N 0.12 0.03 4.6 0.03 0.01 0.003 — — — — — — — — — — — — O 0.12 0.02 4.7 0.02 0.01 0.003 0.7 — — — — — — — — — — — P 0.11 0.03 4.6 0.02 0.01 0.003 — 0.2 — — — — — — — — — — Q 0.12 0.02 4.6 0.03 0.01 0.003 — — 0.002 — — — — — — — — — R 0.11 0.03 4.5 0.04 0.01 0.003 — — 0.002 0.02 — — — — — — — — S 0.12 0.02 4.6 0.02 0.01 0.003 — 0.2 — — 0.2 0.1 — — — — — — T 0.12 0.03 4.7 0.03 0.01 0.003 — — 0.002 — — — 0.01 — — — — — U 0.11 0.02 4.7 0.04 0.01 0.003 — — — — — — 0.04 — — — — — U1 0.12 0.02 4.5 0.02 0.01 0.003 — — — — — — — 0.04 — — — — U2 0.11 0.03 4.6 0.03 0.01 0.003 — — — — — — — — 0.03 — — — U3 0.12 0.03 4.5 0.03 0.01 0.003 — — — — — — — — — 0.02 — — U4 0.11 0.03 4.6 0.02 0.01 0.003 — — — — — — — — — — 0.02 — U5 0.12 0.03 4.7 0.02 0.01 0.003 — — — — — — — — — — — 0.02 XA 0.02 0.03 4.5 0.03 0.01 0.003 — — — — — — — — — — — — XB 0.36 0.02 4.6 0.03 0.01 0.003 — — — — — — — — — — — — XC 0.11 0.59 4.5 0.02 0.01 0.003 — — — — — — — — — — — — XD 0.12 0.02 3.5 0.03 0.01 0.003 — — — — — — — — — — — — XE 0.12 0.02 4.5 1.10 0.01 0.003 — — — — — — — — — — — — XF 0.11 0.03 4.6 0.03 0.11 0.003 — — — — — — — — — — — — XG 0.12 0.02 4.6 0.02 0.01 0.020 — — — — — — — — — — — — XH 0.11 0.02 8.1 0.02 0.01 0.003 — — — — — — — — — — — — Underlined items are outside our scope.

Next, each of the hot-rolled steel sheets and cold-rolled steel sheets obtained as described above was charged into a continuous annealing line. In the continuous annealing line, as shown in Tables 2 and 3, each steel sheet was processed and annealed by controlling the heating rate in a temperature range corresponding to a steel sheet temperature of 450° C. to A° C. (where 500≦A≦600) in an annealing furnace, the concentration of hydrogen in a temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. in the annealing furnace, the transit time of the steel sheet, and the maximum end-point temperature of the steel sheet, followed by water quenching and then tempering at 300° C. for 140 seconds. Subsequently, the steel sheet was pickled such that the steel sheet was immersed in a 40° C. aqueous solution containing 5% by mass sulfuric acid. Some of the steel sheets were electrolytically pickled under current density conditions shown in Tables 2 and 3 by alternating electrolysis such that a specimen was held as an anode and a cathode in that order for 3 seconds, whereby specimens were obtained. The concentration of hydrogen in the annealing furnace other than a region where the concentration of hydrogen was controlled as described above was basically 10% by volume. Gaseous components in an atmosphere were a nitrogen gas, a hydrogen gas, and inevitable impurity gases. The dew point of the atmosphere was controlled by absorbing or removing moisture in the atmosphere. The dew point of the atmosphere was −35° C.

The specimens obtained as described above were measured for TS and El. Furthermore, the specimens were investigated for chemical conversion treatability and corrosion resistance after electrodeposition coating. The amount (internal oxidation amount) of oxides present in a surface portion of each steel sheet, the surface portion being located directly under a surface layer of the steel sheet and being within 100 μm from the surface layer, was measured. Measurement methods and evaluation standards are as described below.

Chemical Conversion Treatability

A chemical conversion solution used was a chemical conversion solution (PALBOND® L3080) produced by Nihon Parkerizing Co., Ltd. Each specimen was subjected to a chemical conversion treatment by a method below. The specimen was degreased with a degreasing solution, FINECLEANER®, produced by Nihon Parkerizing Co., Ltd.; washed with water; surface-modified with a surface modifier, PREPALENE® Z, produced by Nihon Parkerizing Co., Ltd. for 30 seconds; immersed in the chemical conversion solution (PALBOND® L3080) at 43° C. for 120 seconds; washed with water; and then dried with hot air. Randomly selected five fields of view of the specimen subjected to the chemical conversion treatment were observed with a scanning electron microscope (SEM) at 500× magnification. The area fraction of a lack of hiding in a chemical conversion coating was measured by image processing. The specimen was evaluated depending on the area fraction of a lack of hiding as described below. “A” is an acceptable level.

A: 10% or less

B: more than 10%

Corrosion Resistance after Electrodeposition Coating

A test piece with a size of 70 mm×150 mm was cut out of each specimen, obtained by the above method, subjected to the chemical conversion treatment, followed by cationic electrodeposition coating (baking conditions: 170° C. for 20 minutes, a thickness of 25 μm) using PN-150G® manufactured by Nippon Paint Co., Ltd. Thereafter, end portions and a surface not evaluated were sealed with an Al tape and cross cuts (a cross angle of 60°) were made with a cutter knife to reach a base metal, whereby a specimen was prepared. Next, the specimen was immersed in a 5% aqueous solution of NaCl (55° C.) for 240 hours, taken out, washed with water, and then dried, followed by peeling the tape from a cross cut portion. The separation width was measured and evaluated as described below. “A” is an acceptable level.

A: a separation width of less than 2.5 mm per side

B: a separation width of 2.5 mm or more per side

Workability

A JIS #5 tensile test piece was taken from each sample in a 90° direction with respect to a rolling direction and measured for workability such that tensile testing was performed at a constant cross head speed of 10 mm/min in accordance with JIS Z 2241 and the tensile strength (TS/MPa) and elongation (El/%) were determined. A test piece satisfying the inequality TS×El≧18,000 was rated good. A test piece satisfying the inequality TS×El<18,000 was rated poor.

Internal Oxidation Amount in Region within 100 μm from Surface Layer of Steel Sheet

The internal oxidation amount was measured by “impulse furnace fusion-infrared absorption spectrometry.” The amount of oxygen contained in steel (that is, an unannealed high-strength steel sheet) needs to be subtracted. Therefore, surface portions of both sides of each continuously annealed high-strength steel sheet were polished by 100 μm or more, the concentration of oxygen in steel was measured, and the measurement defined as the oxygen amount OH in steel. Furthermore, the concentration of oxygen in steel was measured over the continuously annealed high-strength steel sheet in a thickness direction and the measurement defined as the oxygen amount OI after internal oxidation. The difference between OI and OH (=OI−OH) was calculated using the oxygen amount OI of the high-strength steel sheet, obtained as described above, after internal oxidation and the oxygen amount OH in steel. Furthermore, a value (g/m²) converted into an amount per unit area (that is, 1 m²) per side was defined as the internal oxidation amount.

Results obtained as described above are shown in Tables 2 and 3 together with manufacturing conditions.

TABLE 2 Annealing furnace Concentration Transit of time of hydrogen in steel Heating temperature Maximum sheet in rate in range of end-point temperature Steel temperature 600° C. temperature range of Internal Si Mn Cold range of to 700° C. of steel 600° C. to oxidation Steel (mass (mass or hot 450° C. to (volume A sheet 700° C. amount No. symbol percent) percent) rolling A° C. (° C./s) percent) (° C.) (° C.) (minutes) (g/m²)  1 A 0.02 4.4 Cold 12  5 550 649 1.2 0.096 rolling  2 A 0.02 4.4 Cold 12 12 550 651 1.2 0.055 rolling  3 A 0.02 4.4 Cold 12 19 550 648 1.2 0.031 rolling  4 A 0.02 4.4 Cold 12 20 550 652 1.2 0.029 rolling  5 A 0.02 4.4 Cold 12 24 550 649 1.2 0.020 rolling  6 A 0.02 4.4 Hot 12 25 550 650 1.2 0.018 rolling  7 A 0.02 4.4 Cold 12 36 550 649 1.2 0.011 rolling  8 A 0.02 4.4 Cold 12 51 550 647 1.2 0.006 rolling  5a A 0.02 4.4 Cold 12 24 550 649 0.2 0.021 rolling  5b A 0.02 4.4 Cold 12 24 550 649 0.4 0.020 rolling  5c A 0.02 4.4 Cold 12 24 550 649 0.5 0.019 rolling  5d A 0.02 4.4 Cold 12 24 550 649 3.0 0.020 rolling  5e A 0.02 4.4 Cold 12 24 550 649 10.0 0.019 rolling  5f A 0.02 4.4 Cold 12 24 550 598 1.2 0.020 rolling  5g A 0.02 4.4 Cold 12 25 550 600 1.2 0.021 rolling  5h A 0.02 4.4 Cold 12 25 550 700 1.2 0.019 rolling  5i A 0.02 4.4 Cold 12 24 550 703 1.2 0.021 rolling  9 A 0.02 4.4 Cold  1 24 550 650 1.2 0.019 rolling 10 A 0.02 4.4 Cold  3 25 550 649 1.2 0.017 rolling 11 A 0.02 4.4 Cold  5 23 550 651 1.2 0.015 rolling 12 A 0.02 4.4 Cold  9 24 550 650 1.2 0.017 rolling 13 A 0.02 4.4 Cold 30 25 550 652 1.2 0.019 rolling 14 A 0.02 4.4 Cold 100  25 550 651 1.2 0.021 rolling 15 A 0.02 4.4 Cold 12 24 495 650 1.2 0.015 rolling 16 A 0.02 4.4 Cold 12 25 500 649 1.2 0.016 rolling 17 A 0.02 4.4 Cold 12 24 525 648 1.2 0.019 rolling 18 A 0.02 4.4 Cold 12 24 575 650 1.2 0.017 rolling 19 A 0.02 4.4 Cold 12 25 600 650 1.2 0.018 rolling 20 A 0.02 4.4 Cold 12 25 550 651 1.2 0.017 rolling 21 A 0.02 4.4 Cold 12 24 550 649 1.2 0.018 rolling 22 A 0.02 4.4 Cold 12 23 550 650 1.2 0.020 rolling Corrosion resistance after Current Chemical electro- Electrolytic density conversion deposition TS El No. pickling (A/dm²) treatability coating (MPa) (%) TS × El Workability Remarks  1 Not — B B 1058 22.0 23276 Good Comparative performed Example  2 Not — B B 1052 21.5 22618 Good Comparative performed Example  3 Not — B A 1051 22.4 23542 Good Comparative performed Example  4 Not — A A 1053 21.6 22745 Good Example performed  5 Not — A A 1052 21.9 23039 Good Example performed  6 Not — A A 1055 21.9 23105 Good Example performed  7 Not — A A 1058 21.1 22324 Good Example performed  8 Not — A A 1050 20.8 21840 Good Example performed  5a Not — A A 941 16.4 15432 Poor Comparative performed Example  5b Not — A A 991 18.1 17937 Poor Comparative performed Example  5c Not — A A 1059 21.8 23086 Good Example performed  5d Not — A A 1054 22.0 23188 Good Example performed  5e Not — A A 1056 22.1 23338 Good Example performed  5f Not — B A 1055 21.8 22999 Good Comparative performed Example  5g Not — A A 1053 21.9 23061 Good Example performed  5h Not — A A 1057 21.8 23043 Good Example performed  5i Not — B A 1059 21.4 22663 Good Comparative performed Example  9 Not — B B 1060 21.6 22896 Good Comparative performed Example 10 Not — B B 1050 21.5 22575 Good Comparative performed Example 11 Not — B A 1049 21.5 22554 Good Comparative performed Example 12 Not — A A 1051 21.4 22491 Good Example performed 13 Not — A A 1049 20.7 21714 Good Example performed 14 Not — A A 1047 21.4 22406 Good Example performed 15 Not — B B 1050 21.0 22050 Good Comparative performed Example 16 Not — A A 1054 21.4 22556 Good Example performed 17 Not — A A 1050 21.6 22680 Good Example performed 18 Not — A A 1051 20.9 21966 Good Example performed 19 Not — A A 1059 20.8 22027 Good Example performed 20 Performed 1 A A 1051 21.0 22071 Good Example 21 Performed 3 A A 1049 21.4 22449 Good Example 22 Performed 10 A A 1056 21.2 22387 Good Example Underlined items are manufacturing conditions outside our scope.

TABLE 3 Annealing furnace Concentration of Transit hydrogen Maximum time of in end- steel Heating temperature point sheet in rate in range of temperature temperature Steel temperature 600° C. of range of Internal Si Mn Cold range of to 700° C. steel 600° C. to oxidation Steel (mass (mass or hot 450° C. to (volume A sheet 700° C. amount No. symbol percent) percent) rolling A° C. (° C./s) percent) (° C.) (° C.) (minutes) (g/m²) 23 B 0.02 4.5 Cold 12 25 550 648 1.2 0.017 rolling 24 C 0.02 4.8 Cold 12 23 550 652 1.2 0.016 rolling 25 D 0.11 4.6 Cold 12 24 550 650 1.2 0.017 rolling 26 E 0.31 4.7 Cold 12 25 550 650 1.2 0.019 rolling 27 F 0.49 4.5 Cold 12 25 550 649 1.2 0.021 rolling 28 G 0.02 3.7 Cold 12 24 550 650 1.2 0.018 rolling 29 H 0.03 6.2 Cold 12 24 550 651 1.2 0.017 rolling 30 I 0.02 8.0 Cold 12 25 550 652 1.2 0.019 rolling 31 J 0.02 4.6 Cold 12 24 550 650 1.2 0.020 rolling 32 K 0.03 4.5 Cold 12 25 550 649 1.2 0.021 rolling 33 L 0.02 4.6 Cold 12 25 550 650 1.2 0.020 rolling 34 M 0.03 4.4 Cold 12 23 550 648 1.2 0.019 rolling 35 N 0.03 4.6 Cold 12 24 550 650 1.2 0.018 rolling 36 O 0.02 4.7 Cold 12 25 550 649 1.2 0.017 rolling 37 P 0.03 4.6 Cold 12 23 550 651 1.2 0.019 rolling 38 Q 0.02 4.6 Cold 12 24 550 652 1.2 0.018 rolling 39 R 0.03 4.5 Cold 12 24 550 650 1.2 0.017 rolling 40 S 0.02 4.6 Cold 12 25 550 652 1.2 0.018 rolling 41 T 0.03 4.7 Cold 12 24 550 650 1.2 0.021 rolling 42 U 0.02 4.7 Cold 12 25 550 650 1.2 0.020 rolling 43 U1 0.02 4.5 Cold 12 23 550 651 1.2 0.019 rolling 44 U2 0.03 4.6 Cold 12 25 550 649 1.2 0.018 rolling 45 U3 0.03 4.5 Cold 12 24 550 650 1.2 0.019 rolling 46 U4 0.03 4.6 Cold 12 25 550 650 1.2 0.017 rolling 47 U5 0.03 4.7 Cold 12 24 550 649 1.2 0.018 rolling 48 XA 0.03 4.5 Cold 12 25 550 651 1.2 0.019 rolling 49 XB 0.02 4.6 Cold rolling 12 24 550 650 1.2 0.020 50 XC 0.59 4.5 Cold rolling 12 23 550 650 1.2 0.026 51 XD 0.02 3.5 Cold rolling 12 25 550 651 1.2 0.023 52 XE 0.02 4.5 Cold rolling 12 24 550 651 1.2 0.028 53 XF 0.03 4.6 Cold rolling 12 25 550 650 1.2 0.024 54 XG 0.02 4.6 Cold rolling 12 24 550 649 1.2 0.020 55 XH 0.02 8.1 Cold rolling 12 25 550 650 1.2 0.022 Corrosion resistance after Current Chemical electro- Electrolytic density conversion deposition TS El No. pickling (A/dm²) treatability coating (MPa) (%) TS × El Workability Remarks 23 Not — A A 1044 20.9 21820 Good Example performed 24 Not — A A 1046 21.1 22071 Good Example performed 25 Not — A A 1041 20.8 21653 Good Example performed 26 Not — A A 1043 21.1 22007 Good Example performed 27 Not — A A 1052 21.8 22934 Good Example performed 28 Not — A A 877 23.0 20171 Good Example performed 29 Not — A A 1167 17.2 20072 Good Example performed 30 Not — A A 1205 15.0 18075 Good Example performed 31 Not — A A 1056 21.8 23021 Good Example performed 32 Not — A A 1050 21.0 22050 Good Example performed 33 Not — A A 1052 20.9 21987 Good Example performed 34 Not — A A 1055 20.7 21839 Good Example performed 35 Not — A A 1049 21.5 22554 Good Example performed 36 Not — A A 1050 21.3 22365 Good Example performed 37 Not — A A 1051 20.7 21756 Good Example performed 38 Not — A A 1052 21.0 22092 Good Example performed 39 Not — A A 1058 21.6 22853 Good Example performed 40 Not — A A 1056 21.4 22598 Good Example performed 41 Not — A A 1054 21.6 22766 Good Example performed 42 Not — A A 1050 20.8 21840 Good Example performed 43 Not — A A 1051 20.9 21966 Good Example performed 44 Not — A A 1050 21.4 22470 Good Example performed 45 Not — A A 1053 21.3 22429 Good Example performed 46 Not — A A 1048 21.9 22951 Good Example performed 47 Not — A A 1058 20.5 21689 Good Example performed 48 Not — A A 1354 12.8 17331 Poor Comparative performed Example 49 Not — A A 987 14.6 14410 Poor Comparative performed Example 50 Not — B A 1204 11.5 13846 Poor Comparative performed Example 51 Not — A A 856 22.9 19602 Poor Comparative performed Example 52 Not — B B 1164 20.5 23862 Good Comparative performed Example 53 Not — B A 1196 20.8 24877 Good Comparative performed Example 54 Not — A B 1113 21.6 24041 Good Comparative performed Example 55 Not — B B 1211 14.8 17923 Poor Comparative performed Example Underlined items are manufacturing conditions outside our scope.

As is clear from Tables 2 and 3, high-strength steel sheets manufactured by our method has excellent chemical conversion treatability, corrosion resistance after electrodeposition coating, and workability, although the high-strength steel sheets contain large amounts of oxidizable elements such as Si and Mn. However, in the Comparative Examples, one or more of chemical conversion treatability, corrosion resistance after electrodeposition coating, and workability are inferior.

INDUSTRIAL APPLICABILITY

Our high-strength steel sheet has excellent chemical conversion treatability, corrosion resistance, and workability and can be used as a surface-treated steel sheet for lightening and strengthening automobile bodies. The high-strength steel sheet can be used in various fields such as home appliances and building materials, other than automobiles in the form of a surface-treated steel sheet manufactured by imparting rust resistance to a base steel sheet. 

1-4. (canceled)
 5. A method of manufacturing a high-strength steel sheet, comprising continuously annealing a steel sheet containing, by mass %, 0.03% to 0.35% C, 0.01% to 0.50% Si, 3.6% to 8.0% Mn, 0.01% to 1.0% Al, 0.10% or less P, and 0.010% or less S, the remainder being Fe and inevitable impurities, wherein in a heating step, the steel sheet is heated at a heating rate of 7° C./s or more in a temperature range corresponding to an annealing furnace inside temperature of 450° C. to A° C. (where 500≦A≦600), a maximum end-point temperature of the steel sheet in an annealing furnace is 600° C. to 700° C., transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600° C. to 700° C. is 30 seconds to 10 minutes, and concentration of hydrogen in an atmosphere is 20% by volume or more.
 6. The method according to claim 5, wherein the steel sheet further contains, by mass %, one or more selected from among 0.001% to 0.005% B, 0.005% to 0.05% Nb, 0.005% to 0.05% Ti, 0.001% to 1.0% Cr, 0.05% to 1.0% Mo, 0.05% to 1.0% Cu, 0.05% to 1.0% Ni, 0.001% to 0.20% Sn, 0.001% to 0.20% Sb, 0.001% to 0.10% Ta, 0.001% to 0.10% W, and 0.001% to 0.10% V as a component composition.
 7. The method according to claim 5, further comprising performing electrolytic pickling in an aqueous solution containing sulfuric acid after the continuous annealing is performed.
 8. A high-strength steel sheet manufactured by the method according to claim 5, wherein the amount of an oxide of one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V per side is less than 0.030 g/m² in total, the oxide being formed in a surface portion of the steel sheet that is within 100 μm from a surface of the steel sheet.
 9. The method according to claim 6, further comprising performing electrolytic pickling in an aqueous solution containing sulfuric acid after the continuous annealing is performed.
 10. A high-strength steel sheet manufactured by the method according to claim 6, wherein the amount of an oxide of one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V per side is less than 0.030 g/m² in total, the oxide being formed in a surface portion of the steel sheet that is within 100 μm from a surface of the steel sheet.
 11. A high-strength steel sheet manufactured by the method according to claim 7, wherein the amount of an oxide of one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V per side is less than 0.030 g/m² in total, the oxide being formed in a surface portion of the steel sheet that is within 100 μm from a surface of the steel sheet. 